Positive reversal process



Oct. 16, 1945. H. w. HOUSTON POSITIVE REVERSAL PROCESS Filed April 21,1943 Patented Oct. 16, 1945 OSITIVE REVERSAL PROCESS Herbert W. Houston,Sherman Oaks, Calif., assig'nor to The Houston Corporation, Los Angeles,Caiif., a corporation of California Application April 21, 1943, SerialNo. 483,916

8 Claims.

My invention relates to photographic methods, with special reference tofilm development and is directed specifically to an improved process forproducing positive images by reversal.

In prior art practices heretofore prevalent, it has been consideredessential to develop the exposed film initially to a relatively highgamma or substantially to completion to preclude any possible existenceof a residual latent negative image. Under ordinary and favorableconditions such a process involving initial development to a high gammaresults in a final product of acceptable quality, that is to say, theprocess is satisfactory if the film is given normal or optimum exposurein the camera.

This accepted practice characterized by initial development to arelatively high gamma is not satisfactory over a wide range of exposure.Any degree of over-exposure in the camera causes disproportionate lossin quality in the final positive image, and substantial over-exposureusuallyresults in a completely worthless finished product. Thedisastrous effect of camera over-exposure in conventional reversalprocesses'may be readily understood when it is considered that thedeveloped negative image is in effect a subtraction from the potentialcapacity of the film emulsion to provide density in the final positiveimage. If an excessive proportion of the halide grains in the emulsionis removed, no amount of subsequent exposure of the remaining halidegrains will build up an acceptably dense positive image and nophotoelectric control system can be operative to compensate for thedeficiency.

It is an object of my invention to provide a reversal process effectiveto produce a positive image of quality over a wide range of departurefrom normal camera exposure, it being a special object of the inventionto extend the range in the direction of over-'exposure, thereby toutilize film heretofore regarded as over-exposed beyond retrieve.

Another object of the invention is to provide a reversal process thatwill produce a higher average quality thanheretofore attained whenapplied to the development of numerouspositive reversal films fromvarious sources exposed under diverse circumstances with widely varyingdegrees of skill. It is proposed to provide a reversal process that maybe applied indiscriminatelyv to variously exposed film to provideinherently and automatically compensation for all deviations from normalexposure. I

It is a further object of the invention to provide a reversal processwhich is not critical in subsequentbieaching operation removes all ofthe Originally ct ated halide grains. It has heretofore been assumedthat any residual activated halide grains comprising the latent negativeimage will be developed at a later stage in the reversal process tonegate the final positive image. One object of my invention is toprovide a process that permits incomplete initial development of thepotential negative image without involving the usual penalty ofsubsequently P ng a developed residual negative to the materialdetriment of the final product.

Since initial full development reduces the subsequent image-formingcapacity of the film, a film that is over-exposed in the camera and theninitially developed to completion in the conventional manner isinherently incapable of producing an adequate positive image. One of theobjects of my invention is to achieve an acceptable, if not anexcellent, positive image in the case of over-exposed film by making itpossible to utilize for the final positive image a substantial portionof the halide grains that are initially activated in the camera as partof the initial latent negative image. I have good reason to believe thatin at least one practice of my invention emphasizing the treatment ofover-exposed film. at least some halide grains that are initiallyactivated in the camera and survive temporarily as the residual latentnegative image are actually deactivated in the course of my process andsubsequently respond to light to form density in the final positiveimage.

v A further object of my invention, then, in the practice of myinvention that includes processing of over-exposed film is to provide ableaching agent and a bleaching step that will have the effect oferasing a residuallatent negative image, the term erasing being used inthe sense that the bleach deactivates the grains comprising suchresidual image and restores the grains to lightresponsiveness forsubsequent participation in a positive image. In this regard it is alsoan obj ect of my invention to provide a bleaching agent and a bleachinstep that will have, in effect, a certain selective action in the sensethat the activated grains comprising the residual latent negative imagewill be deactivated and yet other copositive image will survive. v

A still further object of my invention is to provide a procedure-fordeveloping a positive image involving the use of successive lightexposures in r such manner as inherently to provide compensa- "tionfforvarious degrees of camera exposure of The above and. other objects of myinvention will be apparent in the following detailed description,takenwith the accompanying drawing.

In the drawing, which isto be regarded as illustrative only:

Fig. l is a diagra aticview largely in section indicating theconstruction of an apparatus that may be employed in the practice of theinvention; and

Fig. 2 is a similar fragmentary view of a modified form of apparatus.

In the simplest practice of my invention the exposed direct positivefilm is first developed to substantially less than completion and ispassed into a stop solution to arrest development. The film is thenexposed to the light of a printing lamp with the developed negativeimage serving as a printing mask. After the printing exposure, the filmis passed successively through a bleaching solution and a clearingsolution and is then exposed to a second lamp. After the secondexposure. to printing light, the film is developed,

iixed,-washed ,and dried;;in the usual manner.

- In this procedurd'tlre film goes through two cycles of light exposure.Q7111 the first cycle, in which the exposure occurs"'before thebleaching step, a negative image of efiective density serves as a maskto favor the printing of a positive image, and inthe second cycle,additional grains of silver halide are activated-to provide a latentpositive image of adequate density. The final development brings out thepositive image created by the two printing cycles.

The degree to which initial development is curtailed in the creation ofthe initial developed negative image will vary in various practices ofthe invention. Since the purpose of such curtailment is to conserve someof the silver halide grains that are initially 1ight-activated in thecamera, the degree ofcurtailment may be determined in advance more orless precisely in accord with a known need of utilizing in the finalpositive image a portion of the initially light-activated halide grains.If the degree of over-exposure in the "camera is known, such a need maybe closely estimated. A feature of the invention, however, is that it isnot necessary to ascertain in advance what proportion of thecamera-activated grains will be required in a particular instance toavoid deficiency in the final positive image, since curtailment ofdevelopment of the initial negative image suflicient to take care of amaximum requirement in the positive image will meet all lesserrequirements. In other degree w ill mee'tlthe requirements for all filmsthat are .dverxpbsed inithe; camera to lesser degrees.

Under highly favorable conditions the range of deviation from normalcamera exposure may be relatively small, and the need for curtailmentdirection of over-exposure, rather severe curtailment of initialnegative image development will be required. For example, if the processis to be employed for commercial finishing of amateur film from diversesources or is to be employed to finish film taken under war conditionsby the armed services, substantial curtailment wili be found desirable.In my preferred practice, curtailment amounts to development of theinitial negative image to .65-.75 gamma instead of the usual 1.3-1.6gamma.

In my preferred procedure, each printing cycle includes development ofthe film as well as exposure to light, and in each printing cycle amasking image is present on the film. The preferred procedure is asfollows. When the initial negative image is developed to substantiallyless of the initial negative image development cordeparture from normalexposure. especially in the than completion, the film is passed into anacid stop and is then exposed to light from a printing lamp with thenegative image serving as a printing mask. After printing exposure, thefilm passes successively through a bleaching solution and a clearingsolution. Preferably the bleaching solution comprises potassiumdichromate and sulphuric acid. After the film is bleached and cleared,it is subjected to the action of a developjig solution for a period oftime sufllcient to bring out an initial positive image. With thisinitial positive image as a mask or shield covering light sensitivehalide grains, the film is illuminated by a second lamp, which may betermed a flashing lamp and then the film is again developed. Finally,the film is passed through a stop solution and is fixed, washed, anddried in the usual manner.

It is to be noted that in the described preferred procedure a negativeimage acts as a printing mask when the film is exposed to the printinglamp and later a positive image acts as a mask when the film issubsequently exposed to the flashing lamp. The masking'functions of thetwo images are quite different from each other. The negative image actsprimarily as a reversing mask for determining a pattern of response ofhalide grains to light. The masking function of the positive image, onthe other hand, is not that of reversal but rather is that of retardingor preventing the activation of additional halide grains to increase thedensity of the positive image, when such increase is not desirable. Animportant feature of my invention is that the light-restraining effectof the positive image in its function as a mask is automatically andinherently of whatever degree is required by the particular film in viewof the degree to which the film has been originally exposed to light inthe camera. In other words, the positive image brought out by theinitial action of the second developing solution prior to the secondexposure to light serves automatically as means to compensate forvarious degrees of camera exposure and is so effective that automaticprinting control by use of photoelectric cells and other expedients isunnecessary.

The curtailment of the first developing step to bring out the negativeimage at a point substantially short of development to completionnecessarily leaves a residual latent negative image represented by a.number of light-activated halide grains. Immediatel after thedevelopment of the negative image is arrested by the initial stopsolution, the exposure of the film to the printing lamp with thedeveloped negative image serving as a printing mask results in theactivation of a second group of halide grains to coexist with the firstgroup, the first group comprising the residual latent negative image andthe second group comprising the initial latent residual latent negativeimage are not merely deactivated, but are also restored at least tosubstantial extent to their original state of sensitivity to light.

Bleaching solutions ordinarily used in positive reversal processes donot have such selective and restoring effect on the activated halidegrains. Permanganate bleaches, peroxide bleaches, and iodine bleaches,for example, simply destroy both the residual latent negative image andthe initial latent positive image. In my preferred practice I employ adichromate bleach, the proportions being 1% dichromate and /2 of 1%sulphuric acid in water solution. Dichromate of potassium, dichrornateof ammonium, or dichromate of sodium may be used. Another bleach foundto be operative in the present process for selective action on theactivated halide grains is a copper sulphate iron bleach comprising 1%copper sulphate, /2 of 1% sulphuric acid, and 6 of 1% pure iron powder.

I am unable to explain just wh my bleach has the selective effect inerasing the residual latent negative image without destroying the latentpositive image. At first thought there appears to be no basis on whichan explanation of selective action on the two groups of activated grainscould be based. On analysis, however, it appears that there may be somebasis. first. in the fact that the two groups of grains are activatedunder different conditions, and, second, in the fact that the two groupsof grains are subjectto different treatments between the initialactivation of the grains and the contact of the grains with the bleachsolution. Either of these facts may account for an actual difierence incharacter between the two groups.

With reference to the conditions under which the halide grains areactivated, it is apparent that the grains comprising the residual latentnegative image are activated in the camera, whereas the grainscomprising the latent positive image are activated in the course ofprocessing the film and in the presence of whatever products may becreated in the initial developing and initial stopping and may surviveto be present when the grains to comprise the latent positive image areactivated by the printing lamps. It is conceivable that the twodifferent conditions under which light activation occurs may result indefinite distinctions in character of the activated grains.

The possibility of differentiation in character between the two groupsof activated grains occurring after initial activation arises from thefact that the activated grains comprising the residual latent negativeimage are subject to and must survive the initial devloping treatmentand the initial stop treatment. The grains comprising the latentpositive image are activated later and therefore are not subject to suchtreatments after activation. The treatment may definitely change theproperties of the activated grains of the residual latent negativeimage.

An alternative explanation for the apparent selective action withrespect to the two groups of activated grains is that the efiect of thebleach is quantitative rather thanf qualitative. In other words, it isconceivable that the bleaching agent deactivates and restores grains ofboth groups simultaneously, but that a substantial or major portion ofthe activated grains comprising .the latent positive image survivesafter the bleaching action deactivates all of the few grains comprisingthe residual latent negative image.

Since the initial developed negative image functions as a reversal maskand since the effectiveness of the negative image in blocking lightdepends upon the degree to which the film is exposed in the camera, Iadjust the printing lamps for under-exposed film, that is to say, thefirst printing lamp is adjusted to match a relatively weak negativemasking image. When the light so adjusted is applied to a relativelythin negative masking image, the result is a latent positive image ofadequate density. Since, obviously, the density of the initial latentpositive image varies inversely as the density of the negative maskingimage, film that is over-exposed in the camera to yield a stronginitial'negative image will, when exposed to the first printing lamp,produce a weak and inadequate latent positive image.

When, after exposure to the first printing lamp and treatment by thebleach, a film is immersed in the developing solution the latentpositive image develops with a speed that depends upon the strength ofthe latent positive image. Since a film that is under-exposed in thecamera has a strong latent positive image'created by the first printinglamp, such a film will in the subsequent developing solution bring outan adequate positive image in a very short period of time. In thepractice of my invention in which only two lamps are used, a firstprinting lamp prior to bleaching and a second flashing lamp afterbleachin and initial development, the period of time in which thefllm.is immersed in developing solution immediately prior to exposure tothe flashing lamp is sufficient to bring out to adequate density astrong latent positive image on camera under-exposed film. In fact, thisinitially developed positive image on film that is grossly under-exposedin the camera is such an effective mask that the second cycle of lightexposure has little or no effect in activating additional halide grains,and the subsequent or final immersion of the film in the developingsolution has substantially no effect other than expected enrichment ofthe positive image.

Since a weak latent positive image comes out slowly in a developingsolution and cannot be given adequate density, even by forceddevelopment, fiim that is over-exposed in the camera and therefore has arelatively weak latent positive image reaches the second light-exposurewith a positive image that is too thin to serve as a highly efiectivemask. The flashing lamp therefore penetrates the initially developedpositive image to activate additional halide grains for increase indensity of the positive image. Since the film is wet with residualdeveloping solution, additional halide grains turn to silver in thecourse of exposure to the flashing lamp. If the initially developedpositive image needs relatively little increase in density, suchincrease may occur early in the period of exposure to the flashing lamp,the augmented positive image becommally exposed in ing an effective maskto prevent further increase in halide grain activation. If, on the otherhand, the initially developed positive image falls far short of adequatedensity because the film has been grossly over-exposed in the camera,the whole period of exposure to the flashing lamp may be needed foractivation of the required number of additional halide grains. It isapparent that the process automatically compensates for various degreesof camera exposure without the employment of photoelectric control.

In another practice of my invention I add to the preferred proceduredescribed above at least one additional light exposure. The thirdpractice may be carried out as follows. The initial negative image isdeveloped to substantially less than completion in the usual manner, forexampie, to .65-.'75 gamma, whereupon the film is passed into the acidstop. The film is then exposed to the lightof a printing lamp with theinitial developed negative image serving as a mask. After this firstprinting exposure, the film is bleached, cleared, and again submerged ina developing solution to bring out an initial positive image, the periodof submersion being approximately 25% of the total development timerequired for a film that is grossly over-exposed in the camera, whichfilm produces a weak initial latent positive image. With this initialpositive image serving as a mask, the film is flashed by a lamp and isthen developed for approximately 50% of the total development time.Again the film is exposed to a flashing lamp, given the final 25% ofdevelopment time, and is then fixed and dried in the usual manner.

In this third practice of the invention, a film that is grosslyunder-exposed in the camera yields a substantially adequate image in the25% development period and is little affected by the subsequent twoflashing lamps. A film that is northe camera yields a less effectivepositive image in the 25% development period and the positive image isbrought to a final density in the following 50% development period. Afilm that is grossly over-exposed in the camera yields a weak and whollyinadequate positive image in the 25% development period, and adequatedensity is not attained until the last 25% development period, Inpractice the adjustment of the successive lamps is not critical, itbeing necessary merely to avoid too drastic exposure in printing thefirst positive image. Thus the light for printing the initial positiveimage may, if desired, be so mild that in all instances subsequent lightexposure will materially affect the film.

In reviewing the variou's'practices of my inventi'on described above, itis apparent that some of the steps of my process are important forcamera under-exposed film, but unimportant for camera over-exposed film,and, conversely, other steps in the process are important for cameraover-exposed film and not at all vital for camera under-exposed film.Thus, film that is grossly under-exposed in the camera to produce arelatively thin negative masking image yields a strong initial positiveimage, the first or printing lamp being of great importance, and thesecond or flashing lamp being unnecessary. Since, moreover,under-exposure of film in the camera does not seriously deplete theavailable light sensitive;

halide grains in the film, it is not essential that the development ofthe masking negative image be stopped substantiall short of completion.

On the other hand, film that is grossly overexposed ln the camera toyield a relatively dense negative masking image produces a relativelythin initial positive image so that subsequent exposure to the flashinglamp is essential for building up density; also, since cameraover-exposure means depletion of light-sensitive halide grains, thestopping of development substantially short of depletion is of utmostimportance.

It is apparent, then, that each of the described practices of myinvention is the blending of two separate processes, one process beingprimarily for camera under-exposed film and the other process beingprimarily for camera over-exposed film. These two processes may beregarded as subcombinations that may either be brought together asdescribed to make a universal process, or in other practices of myinvention may be used as separate processes simplified and specializedfor either under-exposed film or over-exposed film.

In considering the various practices of my invention described above,there is good reason to believe that certain additional factorscontribute to the success of the process.

One factor arises from the fact that the initial camera exposure of thefilm to produce the initial latent negative image activates a relativelylarge proportion of the silver halide grains in the highlight areas ofthe scene and affects a relatively slight proportion of halide grains inthe shadow areas of the scene. Consequently, in the highlight areasthere are relatively few halide grains left to respond to light and thefew do not include any substantial proportion of highly sensitive halidegrains. On the other hand, in the shadow portions of the scene there arerelatively great concentrations of halide grains remaining for responseto light and these large concentrations include in substantial portionhalide grains that are highly sensitive to light. In other words, thereis not only a quantitative distinction between the high-light areas andthe shadow areas, but also a qualitative distinction, the film havinghigher speed in the shadow areas. Both of the distinctions, quantitativeand qualitative, favor reversal, and even if all of the light-activatedgrains comprising the latent negative image were removed by developmentand bleaching, subsequent exposure of the film to light would produce apositive image.

Another factor contributing to favorable differentiation in the speed ofvarious areas of the film is that the products of development arebromides or iodides, which products are agents restraining theresponsiveness of the film to light. It is believed that these productsof developing the initial masking negative image survive in the areas inwhich they are produced to favor the creation of the reversed orpositive image by reducing the speed of the film in those areas.

It is believed that a further favorable factor may be attributed to thebleaching operationand the particular bleaching agent employed in mypreferred practice. It is thought that silverchromium compounds may beproduced in the bleaching operation to survive with restraining effectin the areas of the film which were formerly negative;

Fig. 1 i dicates, by way of example, the construction of apparatus thatmay be employed for carrying out my preferred process in a continuousmanner. The exposed direct positive film I0 is led from a reel ll over aguide roll I! into an initial developing tank l3, the film in the tanktraversing between an upper gang of rolls l5 and a lower gang of rollsl6 for a time period to carry development to approximately .65-.75

gamma. The film with the developednegative image thereon passes overguide rolls l1 and into a tank It of stop solution to arrestdevelopment. The film is then'guided by rolls i! into the light field ofa first printing lamif 20 with the developed negative image serving as alight mask. After this light exposure, the film passes successivelythrough a tank 2| of bleach solution and a tank 22 containing a clearingsolution.

The bleached and cleared film passes over uide rollers 23 and makes anumber of traverses through a developing solution in a tank 24sufiicient for partial development of an initial positive image. At thisJuncturethe partially developed film is guided by rollers 23 under asecond or fiashing lamp 26 with the developed or partially developedpositive image serving as a mask or shield over the light-sensitivehalide grains. After this second printing exposure the film makessumcient additional traverses in the developing tank 24 for finaldevelopment of the W positive image to adequate density if such finaldevelopment is required and then passes successively through a tank 21containing a stop solution, a tank 28 containing a fixing solution, anda wash tank 30. From the wash tank 23 the film is guided by rollers 31into a suitable drying cabinet generally designated 32, the completelydried film merging from the cabinet to be wound onto a reel 33.

To carry out the third of the practices de+ scribed above, the apparatusshown in Fig. 1 may be modified in the manner indicated by Fig. 2. Theapparatus shown in Fig. 2 is largely identical to the apparatus shown inFig. 1, corresponding numerals being employed to designate correspondingparts.

Fig. 2 dlflers from Fig. 1 in showing a relatively extensive developingtank 35 and in showing, by way of example, two flashing lamps 33 and 31associated with the final development tank instead of the singleprinting lamp 28 in Fig. 1. As iii'dlcated in Fig. 2, the film passes inthe usual manner through the first development tank I3 to develop aninitial negative image, and enters the tank I3 to arrest development ata relatively low gamma. The film with "the initial negative imageserving as a printing mask is then exposed to the first printing lamp 23and is then carried through the bleach solution in the tank 2| andthrough the clearing solution in the'tank 22. The bleached and clearedfilm passes over guide rollers 23 and makes a number of traversesthrough the developing solution in the tank 35 sufilcient for partialdevelopment of a normal image, which development brings out an initialpositive image. .The partially developed film is then guided by rollers23 under the flashing lamp 36. To this point the film has been subjectedto the sametreatment as in the apparatus shown in Fig. 1, the printinglamp 38 in Fig. 2 corresponding to the printing lamp 26 in Fig. 1.

After printing exposure under the lamp 3, the film again makes severaltraverses through the developing solution in the tank 35 and is thenguided by rollers 40 under the flashing lamp 31, the accumulated imageon the film serving as a mask or shield. Again the film makes severaltraverses through the solution in the tank 35 for final development ofthe image if such final development is necessary. Finally, the filmpasses into a stop solution and a fixing solution for finishing in themanner heretofore described.

Since it is difilcult to ascertain precisely why my processis'successful and so advantageous over prior art procedures I am not tobe bound by any explanatory comment herein.

The procedures and apparatus described in detail for the p se ofdisclosure and to teach the principles involved will suggest to thoseskilled in the art various changes and substitutions under my basicconcept, and I reserve the right to all departures from my disclosurethat properly lie within the scope of my appended claims.

I claim as my invention:

1. A reversal process for exposed film, including the steps of:developing the exposed film to a gamma of approximately one-half thatpro-. duced by normal development thereof, thereby.

converting a major portion of the initially lightactivated halide grainsof the film to form a ne ative image and leaving unconverted asubstantial remaining portion of the initially light-activated halidegrains; exposin the film to light with said negative image as a mask toactivate additional halide grains in the film; bleaching out the converd grains comprising said developed negative image; again exposing thefilm to light to activate additional halide grains; developing the filmto convert the light-activated grains therein to form a positive image;and fixing the film.

2. Areversal process for exposed film, including the steps of:under-developing the exposed film to form a negative image ofapproximately one-half the gamma produced by normal development thereof;exposing the film to light with said negative image as a mask toactivate additional halide grains in the film to form a latent positiveimage; bleaching out the converted grains comprising saiddevelopednegative image; treating the film with a developing agent to bring outat least in part said latent positive image; exposing the film to lightwith said developed positive image as a mask, thereby activating anyadditional halide grains in the film necessary to bring said positiveimage up to adequate density; again treating the film with a developingagent; and fixing the film.

3. A reversal process for exposed film, including the steps ofdeveloping the exposed film to a gamma of approximately one-half thatproduced by normal development thereof, thereby converting a majorportion of the initially lightactivated halide grains of the film toform a negative image and leaving unconverted a substantial remainingportion of the initially light-activated halide grains; exposing thefilm to light with said negative image as a mask to activate additionalhalide grains in the film to form a latent positive image; bleaching outthe converted grains comprising said developed negative image; treatingthe film with a developing agent to bring out at least in part saidlatent positive image; exposing the film to light with said developedpositive image as a mask, thereby activatin any additional halide grainsin the film necessary to bring said positive image up to adequatedensity; again treating the film with a developing agent; and fixing thefilm.

4. A reversal process for exposed film, including the steps of:developing the exposed film to a gamma-of approximately one-half thatproduced by normal development thereof, thereby converting a majorportion of the initially lightactivated halide grains of the film toform a negative image and leaving unconverted a substantial remainingportion of the initially light-activated halide grains; subjecting thefilm to light exposure with said negative image as a mask to activateadditional halide grains in the film for a latent positive image, saidexposure being insuflicient to result in a positive image of a desireddensity; bleachin out the converted grains comprising said developednegative image; treating the film with a developing agent to form apositive image; exposing the film to light with said positive image as amask, thereby activating additional halide grains in the film tocompensate for said insufilcient exposure; again treatin the film with adeveloping agent; and fixing the film. I

5. A reversal process for direct positive film varying in degree fromcamera under-exposure to camera over-exposure, including the steps of:under-developing the exposed film to a gamma of approximately 0.65,thereby converting a major portion of the initially light-activatedhalide grains of the film to form a negative image and leavingunconverted a substantial remaining portion of the initiallylight-activated halide grains; subjecting the film to light exposurewith said negative image as a mask to activate additional halide grainsin the film for a latent positive image, said exposure beinginsuilicient to create a latent positive image of adequate density inthe film areas of extreme camera over-exposur but sufilcient for areasof extreme camera under-exposure; bleaching out the converted grainscomprising said developed negative image; treating the film with adeveloping agent to bring out a positive image; subjecting the filmagain to light exposure with said positive image as a mask, therebyactivatin additional halide grains in camera over-exposed areas of thefilm; again treating the film with a developing agent; and fixing thefilm.

'6. A reversal process for grossly over-exposed film, including thesteps of: developing the exposed film to a gamma of approximatelyone-half that produced by normal development thereof, thereby convertinga major portion of the initially light-activated halide grains of thefilm to form a negative image and leaving unconverted a substantialremaining portion or the initially light-activated halide grains;subjecting the film to light exposure with said negative image as a maskto activate additional halide grains in the film for a latent positiveimage; bleaching out said developed negative image and restorin thelight-sensitivity of at least some of said unconverted initiallylight-activated halide grains;

treating the film with a developing agent to bring out an initialpositive image of inadequate denslty; subjecting the film at leastonceagain to light with said initial positive image serving as a mask,thereby activating additional halide grains; at least once againtreating the film with a de. veloping agent; and fixing the film.

7. In "a reversal process for exposed film hav. ing'both cameraover-exposed and camera under-exposed'scenes thereon, including thesteps of: substantially under-developing the film, so that said cameraover-exposed scenes are developed to only a relatively low gamma and toform a visible negative image and an invisible latent image of each 01'said scenes on said film; equally exposing each of said scenes to lightand using said visible negative images of each of said scenes as a maskfor such exposure, so as to form a relatively weak latent positive imageoi said camera over-exposed scenes and a relatively strong latentpositive image of said camera under-exposed scenes; treating aid m witha 26 bleaching agent so as to remove said visible negaei der visible alloi said positive images;

tive ima e from each 01' said scenes and soss to resensitize at least aportion of each 01" said invisible latent images to 1mm; again equallyexposing each of said scenes to light so as to strengthen each of saidpositive images; again developing said film to render visible and t.able en i y and radation an 01' said positive images; and fixing saidfilm.

8. In a reversal process for exposed film having both cameraover-exposed and camera under-exposed scenes thereon, including thesteps of: substantially under-developin the film, so

that said camera over-exposed scenes are developedto only a relativelylow gamma and to form a visible negative image and an invisible latentimage of each of said scenes on said film; equally exposing each of saidscenes to light and using said visible negative images of each or saidscenes as a mask for such exposure, so as to form a relatively weaklatent positive image of said camera over-exposed scenes and arelatively strong latent positive image of said camera un der-exposedscenes; treating said film with a bleaching agent so as to remove saidvisible'negative image from each of said scenes and so as to resensitizeat least a portion of each of said invisible latent images to light;again equally exposing eachb! said scenes to light so as to strengtheneach 01' said positive images; again developing said film to partiallydevelop and renagain equally exposing each oi said scenes to light andusing said visible positive images as a mask forsuch exposure, so as toincrease the exposure of said positive images in inverse proportion tothe density 01 said visible positive images; again developing said filmto increase the density 01' said visible positive images; and fixingsaid film.

, KmBERT W. HOUSTON.

